CRAC Class B (2025) — Questions 411–420

Per the source key, the main disadvantage of FM relative to SSB given here is that FM cannot simultaneously monitor (copy) multiple signals on the same channel — its capture effect means only the stronger signal is heard. (Note: D is also a real characteristic of FM, but the official answer key for this item marks A.)

On VHF/UHF, SSB is conventionally upper sideband (上边带, USB). Lower sideband is used on HF below 10 MHz; suppressed/vestigial sideband are not the standard choices here.

On HF at 14 MHz and above (20 m, 15 m, 10 m, etc.), voice uses upper sideband (USB). Below 10 MHz, lower sideband is used. 14 MHz is the threshold where the convention switches to USB.

On 40 m and lower (7 MHz and below, e.g., 80 m, 160 m), voice uses lower sideband (LSB). This is the standard amateur convention matching the 14 MHz/10 MHz threshold rule.

The sideband split at 10.1 MHz is purely a Convention agreed by amateurs worldwide (a band plan habit), not a propagation effect. B and C are false (propagation is independent of sideband choice), and D is technobabble.

An SSB signal is about 2.4–2.7 kHz wide, so to avoid overlapping nearby stations you should keep at least 2–3 kHz separation. B/C are CW/digital spacings, and D (~6 kHz) is wider than necessary.

SSB is a derivative of AM (it is amplitude modulation with carrier and one sideband suppressed), so it falls under the AM category. Chirp, DSSS, and PSK are distinct (frequency/spread/phase) modulations not classified as AM.

CW means continuous wave (等幅电报) — on/off keying of a constant-amplitude carrier to send Morse code. It is the emission mode, not the Morse code itself (B), nor ASK (C) or FSK (D).

CW occupies only a few hundred Hz (the narrowest), narrower than SSB phone (~2.4–2.7 kHz), FM phone (~12.5 kHz), and ATV (hundreds of kHz). This is why CW is the weak-signal DX favorite.

The real reason is regulatory/bandwidth: FM's wide channel far exceeds the narrow necessary bandwidth for Morse (CW), violating the allocation rule to minimize bandwidth. FM technically can carry Morse (B is false), and C/D are not the official rationale.